The semiconductor manufacturing process generally involves heat treatment of silicon wafers in a heating furnace known as a diffusion or oxidation furnace for oxidation, diffusion and CVD purposes. In heat treating silicon wafers in a diffusion furnace, the wafers are placed in an inner tube such as a liner tube and diffusion tube. The liner tube is generally formed of silicon carbide, quartz or mullite while the diffusion tube is formed of quartz or silicon carbide. During repetitive service of the diffusion furnace, the heating coils surrounding the inner tube undesirably, but inevitably deteriorate and sag into contact with the tube. Upon contact with the heating coils, the tube of silicon carbide tends to crack due to arcing because silicon carbide is electroconductive. This is one of major causes for inefficient production in semiconductor manufacture.
To avoid such arcing or cracking, it is a common practice in the art to provide the inner tubes on the outer surface with insulating layers of alumina or zircon base material. The insulating layers are generally formed by adding a binder to powder alumina, silica, zirconia or zircon, coating the mixture to the tube surface, and baking the coating. This process is often designated baking process.
However, the insulating layers thus baked gradually deteriorate and separate from the underlying substrate for a long period of service, eventually powdering into debris. Once the insulating layers are removed, their insulation is of course lost, leaving the risk of failure of the inner tubes themselves upon contact with the heating coils. When the heating coils are in contact with exposed areas of the inner tube where the insulating layer has been stripped, the inner tube is also in electrical contact with the coils with the danger of electrical conduction or shock to the operator if he or she touches the tube. As the insulating layer deteriorates further, it pulverizes into dust which will disperse in the furnace, causing contamination of the clean room in which the diffusion furnace is placed. Such contamination adversely affects the efficient production of semiconductor wafers, leading to low productivity of the semiconductor manufacturing process. The furnace inner tube having an insulating layer formed by the baking process has various problems resulting from deterioration of the insulating layer during an extended period of operation.
The present invention has been made to solve the above-mentioned problems and its object is to provide an inner tube such as a liner tube and diffusion tube for use in semiconductor diffusion furnaces, which inner tube enables consistent heat treatment on semiconductor for an extended period of time without deterioration or separation of the insulating layer.